Method for continuous production of high quality graphene

ABSTRACT

A continuous method for manufacturing graphene films using a metal substrate, wherein a first surface of the metal substrate is heated such that a top layer of the first surface melts to form a molten metal layer, and devices for carrying out the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. patentapplication Ser. No. 15/457,763, entitled “Method for ContinuousProduction of High Quality Graphene,” filed on Mar. 13, 2017, whichclaims the benefit of U.S. Provisional Patent Application No.62/310,350, entitled “Method for Continuous Production of High QualityGraphene,” filed on Mar. 18, 2016, the contents of which are expresslyincorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present disclosure relates to a continuous method for manufacturinghigh quality graphene films using metal substrates.

BACKGROUND

Graphene, an allotrope of carbon forming a two-dimensional,atomic-scale, hexagonal lattice, has become useful for applications inflexible electronics due to its exceptional electrical, mechanical, andchemical properties. In order to realize practical applications ofgraphene, large-scale production methods such as chemical vapordeposition (CVD) on transition metal surfaces have been increasinglyexplored. In particular, copper has become a popular catalytic substratedue to its low carbon solubility at typical growth temperatures.

However, it has been observed that a strictly two-dimensional graphenesystem is often thermodynamically unstable, and frequently exists onlythrough perturbations in a third direction. These fluctuations in thethird direction generally result in a crumpled topography of thegraphene sheet surface, such as “ripples” thereon. It is currentlyunderstood that graphene ripples may be associated with (a) the problemof thermodynamic stability of two-dimensional layers or membranes; (b)the thermal expansion coefficient difference between a metal substrateand graphene; and/or (c) the presence of grain boundaries on the metalsubstrate. As graphene surface topography has significant impact on itsmechanical, electronic, magnetic, and chemical properties, understandingand controlling the formation of ripples is important for exploiting itsexcellent properties.

While continuous production methods for preparing a graphene sheet (suchas roll-to-roll methods, where vapor containing carbon reacts on ahorizontal substrate such as a copper foil) dramatically reducesproduction price, such methods often correspond with unwanted wrinklesor ripples on the grown graphene surface.

There is thus a need in the art for methods of manufacturing highquality and large surface area graphene, while reducing both productionprice and unwanted surface topography.

SUMMARY OF THE INVENTION

The present disclosure relates generally to preparing a film comprisinggraphene, comprising: placing a metal substrate into a processingchamber comprising one or more heating elements and one or more gassupply components; heating the metal substrate to form a molten metallayer on a first surface of the metal substrate, wherein the firstsurface is a top surface; contacting the molten metal layer with acarbon source gas, such as a gas comprising hydrocarbon, to form agraphene-comprising layer substantially covering the molten metal layerof the first surface of the metal substrate; solidifying the moltenmetal layer; and optionally separating the graphene-comprising layer toform a film comprising graphene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a processing chamber, in accordance withaspects of the present disclosure.

FIG. 2 shows a scanning electron microscope (SEM) image of graphenegrains on a copper foil substrate without a molten top layer.

FIG. 3 shows an SEM image of graphene grains on a copper foil substratewith a molten top layer.

FIG. 4 shows an example of a processing chamber, in accordance withaspects of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure relates generally to continuous methods forproducing graphene films on the surface of a metal substrate. Accordingto some aspects, the method comprises a roll-to-roll system. As usedherein, “roll-to-roll” processing refers production continuous methodsutilizing a roll or rolls of flexible metal, such as a metal foil. Thepresent disclosure is also directed to devices for carrying out thesemethods.

According to some aspects, the method of the present disclosure maycomprise placing a metal substrate into a processing chamber. In someaspects, the metal substrate comprises a flat surface, and the metalsubstrate may be a sheet. The metal substrate comprises one or moremetals. Examples of metals which may be useful according to the presentdisclosure include copper, nickel, ruthenium, rhodium, aluminum, andalloys and/or mixtures thereof. According to some aspects, the metalsubstrate may be placed into the processing chamber by advancing themetal substrate (for example, a sheet of metal foil or a metal film)through the processing chamber.

For example, the metal substrate may comprise a metal foil. According tosome aspects, the metal foil may have a thickness of between about 1 and100 μm, preferably between about 1 and 50 μm, more preferably betweenabout 10 and 40 μm, and most preferably between about 20 and 30 μm.According to some aspects, the metal foil may have a thickness of about25 μm.

According to some aspects, the metal substrate may comprise a metalfilm. The metal film may optionally be provided with a second substrate.For example, the metal film may be deposited on one or more surfaces ofthe second substrate. According to some aspects, the second substratemay comprise a metalloid and/or an oxide thereof, for example, Si and/orSiO₂. According to some aspects, the metal film may have a thickness ofbetween about 100 and 1000 nm, preferably between about 100 and 800 nm,more preferably between about 200 and 600 nm, and most preferablybetween about 300 and 500 nm.

According to some aspects, the metal substrate is continuously advancedthrough the processing chamber.

According to some aspects, the processing chamber may comprise, forexample, one or more rolls configured to advance the substrate throughthe processing chamber. For example, the processing chamber may compriseone or more unwinding rolls, one or more guide rolls, and/or one or morewinding rolls. According to some aspects, the unwinding roll(s) may be aroll from which the substrate is unwound, and/or which adjusts thetensile force of the substrate. According to some aspects, the guideroll(s) may transport the film through the processing chamber. Forexample, according to some aspects, the chamber may comprise a firstguide roll proximal to the unwinding roll, and a second guide rollproximal to the winding roll, wherein one or both of the guide rolls isrotatably driven, for example, by a drive source. According to someaspects, the winding roll may be a roll on which the substrate is woundafter graphene has been formed thereon and/or after it has advancedthrough the processing chamber.

According to some aspects, the processing chamber comprises one or moreheating elements and one or more gas supply components. In someembodiments, the processing chamber may comprise an inert atmosphere.Example inert gases for use in the processing chamber include argon,helium, nitrogen, mixtures thereof, and any other inert gases or gasmixtures known in the art.

As used herein, the term “heating element” refers to any element capableof producing heat. According to some aspects, the processing chamber maycomprise a heating element which provides heat to only one surface or tomore than one surface of the metal substrate as it advances through theprocessing chamber. According to some aspects, the processing chambermay comprise at least two heating elements, wherein a first heatingelement or elements provides heat to a first surface of the metalsubstrate, and a second heating element or elements provides heat to asecond surface of the metal substrate, such as a bottom surface. Forexample, a first heating element or elements may be positioned above themetal substrate to provide heat to a top surface of the substrate, and asecond heating element or elements may be positioned below the metalsubstrate to provide heat to a bottom surface of the substrate.According to some aspects, the two or more heating elements may compriseseparate controllers such that the each heating element may beseparately controlled. For example, each heating element may comprise acontroller which controls the heat provided by the heating elementand/or a controller which controls the position of the heating element,for example, the position of the heating element relative to thesubstrate. According to some aspects, one or more heating elements maybe controlled using the same controller. For example, the first heatingelement or elements may be controlled using a first controller while thesecond heating element or elements may be controlled using a secondcontroller.

According to some aspects, the processing chamber may comprise multipleheating elements and/or multiple types of heating elements. According tosome aspects, the heating element(s) may comprise an IR heater, forexample, a heater comprising an IR lamp, and/or a laser heater, forexample, a heater comprising a laser. According to some aspects, theheating element(s) may comprise a ceramic heater, a halogen lamp, alaser or the like, a means for heating by an induced current byapplication of a magnetic field, and/or a means for heating by heatconduction. According to some aspects, the heating element(s) maycomprise a graphite heating element. However, it will be appreciatedthat any suitable heating element capable of providing controlled heatmay be used according to aspects of the present disclosure.

According to some aspects, the method of the present disclosure maycomprise heating the metal substrate as it continually advances throughthe processing chamber. For example, as shown in FIG. 1, using rotatablydriven guide rolls (9 and 10), a sheet of the substrate (1) may beadvanced from an unwinding roll (8) into and through a processingchamber (2) with one or more heating elements (3) therein. According tosome aspects, the one or more heating elements (3) may provide acontrolled heat to the top surface of the substrate (1) as it advancesthrough the processing chamber (2). According to some aspects, theheating element(s) (3) may provide a heat that is equal to or above themelting point of the substrate (1) or the surface thereof. For example,the heating element(s) (3) may provide heat that is greater than themelting point of copper, and/or greater than the melting point of thesurface of a copper substrate. According to some aspects, the substratemay be wound onto the winding roll (11) after graphene has been formedtherein.

It will be appreciated that the heat supplied by the one or more heatingelements to the top surface of the substrate will depend on the meltingpoint of the specific substrate used, and/or the surface melting pointof the substrate used. As used herein, the term “melting point” refersto the temperature at which a solid becomes a liquid at a fixedpressure. According to some aspects, the surface of the substrate maymelt at a temperature which is lower than the melting point of its bulk,allowing surface melting at temperatures which are lower than themelting point of the specific metal and/or alloys and/or mixturesthereof comprised by substrate. For example, according to some aspects,surface melting may occur such that a molten surface layer may grow to athickness of about 10 Å at just below the melting point of the of thespecific metal and/or alloys and/or mixtures thereof comprised bysubstrate. According to some aspects, the melting point of the substratesurface(s) may at least partly depend on the crystal orientation of thesubstrate.

According to some aspects, the heat supplied to the top surface of thesubstrate by the heating element(s) may be varied by varying the amountheat output by the heating element(s), and/or the distance between theheating element(s) and the top surface of the substrate. According tosome aspects, the heat supplied by the one or more heating elements tothe top surface of the substrate may heat the substrate surface to aboutthe same temperature as the melting point of the substrate and/orsubstrate surface, or within a certain range of the melting point of thesubstrate and/or substrate surface. For example, the heat supplied bythe heating element(s) to the top surface of the substrate may heat thesubstrate to no more than about 50° C. different than the melting pointof the substrate and/or substrate surface, preferably no more about 40°C., even more preferably no more than about 30° C., even more preferablyno more than about 20° C., even more preferably no more than about 10°C., and most preferably no more than about 5° C. different than themelting point of the substrate and/or substrate surface.

According to some aspects, the melting point of the substrate and/orsubstrate surface may depend on one or more factors. For example, themelting point of the substrate and/or substrate surface may at leastpartly depend on particular process conditions (for example, thecomposition, pressure, and/or density of the atmosphere in theprocessing chamber) and/or the specific metal and/or alloys and/ormixtures thereof comprised by substrate. According to some aspects ofthe disclosure, the melting point of the substrate and/or substratesurface may be different than the conventional melting point of thespecific metal and/or alloys and/or mixtures thereof comprised bysubstrate.

According to some aspects, the atmosphere of the processing chamber mayhave a predetermined pressure. For example, according to some aspects,the pressure may be between about 10⁻⁸ atm and 100 atm, optionallybetween about 10⁻⁷ atm and 50 atm, and optionally between about 10⁻⁷ atmand 20 atm. According to some aspects, the pressure may be about 0.0084atm.

According to some aspects, the atmosphere of the processing chamber maycomprise a certain gas composition. For example, according to someaspects, the atmosphere may comprise an inert gas. As used herein, theterm “inert” refers to a substance that is not chemically reactive. Forexample, the atmosphere may comprise an inert gas that reacts withneither the substrate nor the carbon source gas. According to someaspects, the atmosphere may additionally or alternatively comprisegasses that are not inert. Examples of gasses that may be comprised bythe atmosphere of the processing chamber include, but are not limitedto, argon, helium, nitrogen, hydrogen, and mixtures thereof.

According to some aspects of the present disclosure, the heatingelement(s) may provide a controlled heat to the top surface of thesubstrate such that the top surface of the substrate melts, thereby atleast partially smoothing any grain boundaries, wrinkles, and/or otherimperfections on the top surface of the substrate. According to someaspects, the heating element(s) may output heat at a temperature ofand/or heat the top surface of the substrate to a temperature of about970° C.±30° C. and/or 1050° C.±30° C. and/or 1100° C.±30° C. Accordingto some aspects, the heating element(s) may output heat at a temperatureof and/or heat the top surface of the substrate to a temperature ofabout 527° C., 627° C., 727° C., 997° C., 1017° C., 1027° C., or 1037°C.

According to some aspects, as the metal substrate advances through theprocessing chamber, a molten metal layer may form on a first surface ofthe metal substrate, wherein the first surface is the top surface. Forexample, a top layer of the metal substrate may melt to form a moltenmetal layer as the surface of the substrate is subjected to heatprovided by the heating element(s). According to some aspects, theamount of melting is controlled such that the resultant molten metallayer has a predetermined depth. The depth of the molten metal layer maybe controlled, for example, by adjusting the heat provided by theheating element(s), the distance between the heating element(s) and thesubstrate, and/or the rate at which the substrate advances through theprocessing chamber.

According to some aspects, at least one additional heating element maybe provided in order to control the temperature of a second surface ofthe metal substrate, for example, the bottom surface of the metalsubstrate or the opposite surface of the metal substrate. For example,the processing chamber may comprise one or more second heating elementsunderneath the substrate, wherein the second heating element(s) maysupply heat such that the temperature of the bottom surface of thesubstrate is kept below the melting point of the substrate and/orsubstrate surface. That is, the second heating element(s) may supplyless heat to the substrate than the first heating element(s). Forexample, the amount of heat output by the first heating element(s) maybe greater than the amount heat output by the second heating element(s)and/or the distance between the first heating element(s) and the topsurface of the substrate may be less than the distance between thesecond heating element(s) and the bottom surface of the substrate. Inanother example, the amount of heat output by the first heatingelement(s) may be about the same as the amount heat output by the secondheating element(s), while the distance between the first heatingelement(s) and the top surface of the substrate may be less than thedistance between the second heating element(s) and the bottom surface ofthe substrate. In this way, the second heating element(s) may stillsupply less heat to the substrate than the first heating element(s).

As discussed herein, according to some aspects, the melting point of thesubstrate and/or substrate surface may depend on one or more factors.According to some aspects, the second heating element(s) may control thesecond substrate surface temperature to be below a temperature of about1050° C. According to some aspects, the heating element(s) may outputheat at a temperature of and/or heat the second surface of the substrateto a temperature of about 970° C.±30° C. and/or 1050° C.±30° C. and/or1100° C.±30° C. According to some aspects, the heating element(s) mayoutput heat at a temperature of and/or heat the second surface of thesubstrate to a temperature of about 527° C., 627° C., 727° C., 997° C.,1017° C., 1027° C., or 1037° C. According to some aspects, the secondsurface of the substrate does not melt, thus a molten metal layer onlyforms on one surface of the substrate as it advances through theprocessing chamber.

According to some aspects, the heat supplied to the bottom surface ofthe substrate by the second heating element(s) may be varied by varyingthe amount heat output by the second heating element(s), and/or thedistance between the second heating element(s) and the bottom surface ofthe substrate. According to some aspects, the heat supplied to thebottom surface of the substrate by the second heating element(s) may atleast partially depend on the presence of one or more spacers orcomponents between the second heating element(s) and the bottom surfaceof the substrate.

For example, as shown in FIG. 4, the processing chamber may comprise asolid spacer (17) underneath at least a portion of the substrate (16)such that at least the portion of the substrate (16) is separated fromthe second heating element(s) (20) by the spacer. According to someaspects, the spacer may comprise a solid platform or scaffold. Accordingto some aspects, the spacer may transmit some or all of the heatsupplied by the heating element(s). According to some aspects, theprocessing chamber may comprise at least one temperature sensor, forexample, a thermocouple. For example, the processing chamber maycomprise a first thermocouple (15) and a second thermocouple (14) inorder to measure the temperature of the surface of the substrate (16)facing the first heating element(s) (18) and the second heatingelement(s) (20), respectively.

According to some aspects, the method comprises contacting the moltenmetal layer or layers with a carbon source gas. According to someaspects, the carbon source gas is a gas comprising hydrocarbon. Thecarbon source gas may be provided using one or more gas supplycomponents which are contained in the processing chamber. The one ormore gas supply components refers to any mechanism that can provide agas (e.g., a carbon source gas, such as a gas which comprises one ormore hydrocarbons) and/or is capable of depositing the gas onto a moltenmetal layer of the metal substrate.

According to some aspects, depositing of the carbon source gas onto thesurface of the substrate may be accomplished using chemical vapordeposition (CVD). Example carbon source gases for use in the processingchamber include methane, ethylene, acetylene, ethanol, benzene,methanol, carbon monoxide, carbon dioxide, carbon-based polymer, anano-carbon material, mixtures thereof, and/or any other gases or gasmixtures known in the art. According to some aspects, the contacting ofthe molten metal layer with a carbon source gas allows for the formationof a graphene-comprising layer. According to some aspects, thegraphene-comprising layer substantially covers or completely covers theportion of the surface of the molten metal layer that is contacted withthe carbon source gas. “Substantially cover” refers to more than 50%,preferably more than 70%, more preferably more than 80%, even morepreferably more than 90%, and even more preferably more than 95% of thesurface area of the surface is covered. In some aspects, the surface iscompletely (about 100%) covered. For example, as shown in FIG. 1, theprocessing chamber may one or more comprise a gas supply components (4)which emits a hydrocarbon gas capable of forming graphene on the surfaceof the substrate via CVD. As shown in FIG. 1, according to some aspects,the one or more gas supply components (4) may provide a hydrocarbon gasto a first surface of the substrate, that is, the surface of thesubstrate with the molten metal layer formed thereon.

In addition to the carbon source gas, the one or more gas supplycomponents may provide one or more secondary gasses. Examples ofsecondary gasses for use in the processing chamber include argon,helium, nitrogen, hydrogen, and mixtures thereof. For example, the oneor more gas supply components may provide the carbon source gas to theprocessing chamber along with the one or more secondary gasses as atransport gas. Alternatively or additionally, the one or more secondarygasses may be provided before and/or after the carbon source gas hasbeen provided to the processing chamber. According to some aspects, theone or more secondary gasses may be provided to the processing chambervia a different gas supply component than the gas supply component(s)used to provide the carbon source gas. For example, the processingchamber may comprise one or more secondary gas supply components whichprovide the one or more secondary gasses to the processing chamber.

According to some aspects, the gas supply component(s) and/or thesecondary gas supply component(s) may provide their respective gasses ata certain flow rate, which may be the same or different for each gas.For example, according to some aspects, nitrogen gas may be provided ata flow rate of about 200 sccm, argon gas may be provided at a flow rateof 600 about sccm, hydrogen gas may be provided at a flow rate of about40 sccm, and methane may be provided at a flow rate of about 10 sccm.According to some aspects, the flow rates of the carbon source gasand/or the one or more secondary gasses into the processing chamber maybe selected in order to provide a certain flow rate ratio. For example,the flow rate ratio of nitrogen gas to argon gas to hydrogen gas tomethane may be about 20:60:40:1

The gas supply component(s) and/or the secondary gas supply component(s)may be arranged such that they emits their respective gassescontemporaneously with the one or more heating elements providing heatto the substrate. Alternatively or additionally, one or more heatingelements may provide heat to the substrate prior to the introduction ofthe gasses from the gas supply component(s) and/or the secondary gassupply component(s). For example, the processing chamber may comprise apreheating zone wherein one or more heating elements provides heat tothe metal substrate in order to form a molten metal layer with a desiredthickness, as described herein. The substrate may then continue to aprocessing zone of the processing chamber comprising the gas supplycomponent(s) and/or the secondary gas supply component(s) and/or one ormore additional heating elements as described herein. Alternatively oradditionally, the preheating zone may comprise one or more secondary gassupply components in order to provide one or more secondary gasses. Forexample, according to some aspects, the preheating zone may comprise oneor more secondary gas supply components that provide hydrogen gas to theprocessing chamber, while the processing zone may comprise one or moregas supply components that provide a carbon source gas along with atransport gas comprising argon, helium, and/or nitrogen.

According to some aspects, the method of the present disclosure allowsdirect synthesis of a uniform layer of graphene directly on the moltenmetal layer of the substrate. For example, as shown in FIG. 1, the solidsubstrate (5) may comprise a molten metal layer (6) on one surface, witha uniform layer of graphene (7) formed thereon. In some preferredembodiments, this direct synthesis reduces and/or eliminates the grainboundaries that are observed when using, for example, solidpolycrystalline copper. According to some aspects, the resultantgraphene layer thus comprises a uniform nucleation distribution and lowgraphene nucleation density.

According to some aspects, the substrate may advance through theprocessing chamber at a predetermined rate. For example, the substratemay advance through the processing chamber at a rate which provides thedesired amount of graphene growth. For example, the rate may be selectedin order to provide a certain growth time (i.e., a certain amount oftime the substrate is in contact with the carbon source gas). Accordingto some aspects, the rate may be selected in order to provide about 30minutes of growth time, optionally about 20 minutes of growth time,optionally about 10 minutes of growth time, and optionally about 5minutes of growth time. It should be understand that the growth timerequired to provide a desired amount of graphene growth may at leastpartly depend on particular process conditions (for example, thecomposition, pressure, and/or density of the atmosphere in theprocessing chamber) and/or the specific metal and/or alloys and/ormixtures thereof comprised by substrate.

According to some aspects, the heat provided by the one or more heatingelement(s) and/or the rate at which the substrate advances through theprocessing chamber may be selected in order to achieve a predeterminedmolten metal layer thickness. The molten metal layer may have apredetermined thickness that corresponds, for example, to a thickness ofthe substrate. For example, according to some aspects, if the substrateis about 25 μm thick (e.g., a copper or copper alloy foil having athickness of about 25 μm), the predetermined molten metal layerthickness may be between about 100 and 1000 nm, preferably between about200 and 800 nm, more preferably between about 300 and 700 nm, and mostpreferably between about 450 and 550 nm. According to some aspects, thepredetermined molten metal layer thickness may be achieved prior to thesubstrate advancing out of the preheating zone of the processing chamberand/or may be maintained in the processing zone of the processingchamber.

According to some aspects, the predetermined molten metal layerthickness may correspond to the critical Marangoni number, which isrelated to Benard-Marangoni convection. In particular, a molten metalmay show solutal or thermal instabilities driven by variations insurface tension, which can be described for instance by Benard-Marangoniconvection. According to some aspects, the thickness of the molten metallayer may be selected such that the solutal Marangoni number for thesurface-melted metal-carbon system is lower than the critical numberM_(a) ^(s)<Mc=80, which prevents the convection flow in the melted layerand thereby prevents formation of cellular ripples on the surface of themelted layer. As it is used herein, the “solutal Marangoni number”refers to:

$M_{a}^{s} = \frac{\gamma_{c}d\; \Delta \; c}{\mu \; D_{L}}$

where γ_(c) is solutal surface tension coefficient, Δc is theconcentration gradient across the melt (Δc=c_(LG)−c_(SL), c_(SL) beingthe bulk concentration at the solid-liquid interface and c_(LG) beingthe concentration at the liquid-gas interface), d is the thickness ofthe molten metal layer, D_(L) is the molecular diffusivity in theliquid, and μ is the dynamic viscosity.

According to this equation, γ_(c), i.e., the solutal surface tensioncoefficient, can be described as:

$\gamma_{C} = \frac{d_{\sigma}}{d_{0}}$

which is defined through the surface tension as:

σ=σ₀+γ_(C)({tilde over (c)}−c ₀),

where {tilde over (c)} is the excess solute concentration on liquid-gasinterface, and σ₀ and c₀ are reference positive values. In the referencesteady state, the fluid is at rest with constant excess soluteconcentration c₀ and the unperturbed bulk concentration c is given by:

c=c _(SL)−β_(Z),

where c_(SL) is bulk concentration at the solid-liquid interface and βis the bulk concentration gradient and is positive when the solute fluxis from solid to liquid and negative if the solute is absorbing from thegas phase. In the case of positive values, the solutal surface tensioncoefficient can be described by:

$\gamma = \frac{\sigma - \sigma_{0}}{\overset{\sim}{c} - c_{0}}$

According to some aspects, the molten metal layer thickness may beselected such that variations in surface tension between the moltenmetal layer and the carbon layer, described for instance byBenard-Marangoni convection, may be minimized. In this way, theformation of domains and/or cells based on solutal or thermalinstabilities, typically observed when using a liquid layer of metalwith an adsorbate on the surface thereof, may be reduced.

According to some aspects, after the formation of thegraphene-comprising layer, the molten metal layer may be solidified.According to some aspects, the method of the present disclosurecomprises advancing the sheet of metal substrate with the graphene layerformed thereon out of the processing chamber such that the molten metallayer solidifies outside of the processing chamber. For example, whenthe sheet of metal substrate is no longer subjected to heat from theheating element(s) in the processing chamber, the substrate may coolsuch that the melted layer thereon solidifies. According to someaspects, the molten metal layer may solidify after the substrate isadvanced out of the processing chamber. According to some aspects, themolten metal layer may be cooled inside the processing chamber, forexample, in a cooling zone of the processing chamber, and may thussolidify before the substrate exits the processing chamber.

According to some aspects, the method of the present disclosure may alsocomprise separating the graphene-comprising layer from the remainder ofthe metal substrate. According to some aspects, the graphene-comprisinglayer is separated from the molten metal layer and remainder of themetal substrate. The graphene-comprising layer may be separated orremoved using any means known in the art. For example, the underlyingmetal substrate (including the solidified molten layer) may be removedusing chemical etching, electrochemical etching, and/or mechanicalremoval in order to produce a high quality, uniform, large surface arealayer comprising graphene.

The present disclosure also relates generally to devices for carryingout the above methods. For example, devices of the present disclosuremay comprise a processing chamber comprising an input, at least oneheating element, at least one gas supply component, and an output,wherein the device is configured to allow a sheet or film of a metalsubstrate to be advanced therethrough.

While the aspects described herein have been described in conjunctionwith the example aspects outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent to those having at least ordinary skill in the art.Accordingly, the example aspects, as set forth above, are intended to beillustrative, not limiting. Various changes may be made withoutdeparting from the spirit and scope of the disclosure. Therefore, thedisclosure is intended to embrace all known or later-developedalternatives, modifications, variations, improvements, and/orsubstantial equivalents.

Thus, the claims are not intended to be limited to the aspects shownherein, but are to be accorded the full scope consistent with thelanguage of the claims, wherein reference to an element in the singularis not intended to mean “one and only one” unless specifically sostated, but rather “one or more.” All structural and functionalequivalents to the elements of the various aspects described throughoutthis disclosure that are known or later come to be known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the claims. Moreover, nothingdisclosed herein is intended to be dedicated to the public regardless ofwhether such disclosure is explicitly recited in the claims. No claimelement is to be construed as a means plus function unless the elementis expressly recited using the phrase “means for.”

Further, the word “example” is used herein to mean “serving as anexample, instance, or illustration.” Any aspect described herein as“example” is not necessarily to be construed as preferred oradvantageous over other aspects. Unless specifically stated otherwise,the term “some” refers to one or more. Combinations such as “at leastone of A, B, or C,” “at least one of A, B, and C,” and “A, B, C, or anycombination thereof” include any combination of A, B, and/or C, and mayinclude multiples of A, multiples of B, or multiples of C. Specifically,combinations such as “at least one of A, B, or C,” “at least one of A,B, and C,” and “A, B, C, or any combination thereof” may be A only, Bonly, C only, A and B, A and C, B and C, or A and B and C, where anysuch combinations may contain one or more member or members of A, B, orC. Nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the present invention, and are not intended to limit thescope of what the inventors regard as their invention nor are theyintended to represent that the experiments below are all or the onlyexperiments performed. Efforts have been made to ensure accuracy withrespect to numbers used (e.g. amounts, dimensions, etc.) but someexperimental errors and deviations should be accounted for.

In particular, the following comparative examples (Example 1 and 2) wereconducted in order to compare processes of growing graphene on a 15 μmthick copper foil substrate using cold wall CVD (i.e., a CVD methodwherein the substrate is heated but the walls of the CVD chamber arecooled) wherein the top and bottom surfaces of the substrate were heatedto a certain temperature. The processes were performed using an AixtronBlack Magic CVD system, which comprises two graphite heaters, one aboveand one below the substrate, which can be controlled separately. Bothprocesses were performed using a pressure of 500 mbar (375 torr) and agas flow of Ar at 3000 sccm, H₂ at 500 sccm, and N₂ at 500 sccm (forsystem cooling). The copper foil substrate was separated from the bottomheater using a quartz spacer. In both examples, the substrate was heatedfor 10 minutes, and then subjected to a flow of methane gas at a flowrate of 10 sccm as a hydrocarbon source.

Example 1

A copper foil substrate was placed into the Aixtron Black Magic CVDsystem. The top heater was kept at a constant temperature of 1100° C.,and the bottom heater was kept at a constant temperature of 1150° C. Thetop and bottom surface temperatures of the substrate were modified bychanging the distance between the substrate surfaces and the top andbottom heaters, respectively. The top surface of the substrate washeated to a temperature of 980° C. (i.e., the temperature used inconventional methods wherein the copper surface is not melted), and thebottom surface was kept at a temperature of less than 1150° C. Thebottom surface of the substrate was separated from the bottom heater bythe quartz spacer so that the bottom layer of the substrate also did notmelt.

Graphene was grown on the top surface of the substrate, and theresultant layer of graphene was visualized using an SEM as shown in FIG.2. FIG. 2 shows that the graphene layer comprised small grains (12),indicating the presence of grain boundaries on the substrate surface(i.e., a rough substrate surface).

Example 2

A copper foil substrate was placed into the Aixtron Black Magic CVDsystem. The top heater was kept at a constant temperature of 1100° C.,and the bottom heater was kept at a constant temperature of 1150° C. Thetop and bottom surface temperatures of the substrate were modified bychanging the distance between the substrate surfaces and the top andbottom heaters, respectively. The top surface of the substrate washeated to a temperature of 1050° C., and the bottom surface was kept ata temperature of less than 1150° C. The bottom surface of the substratewas separated from the bottom heater by the quartz spacer so that thebottom layer of the substrate also did not melt.

Graphene was grown on the top surface of the substrate, and theresultant layer of graphene was visualized using an SEM as shown in FIG.3. FIG. 3 shows that the graphene layer comprised much larger grains(13) than shown in Example 1, indicating the surface of the coppersubstrate had likely been melted and thus, was much smoother.

What is claimed is:
 1. A continuous method for preparing a filmcomprising graphene, the method comprising: providing a metal substrate;continuously advancing the metal substrate into and through a processingchamber; heating the metal substrate to form a molten metal layer on atop surface of the metal substrate; contacting the molten metal layerwith a carbon source gas to form a graphene-comprising layersubstantially covering the molten metal layer of the top surface of themetal substrate; solidifying the molten metal layer; advancing the metalsubstrate having the graphene-comprising layer out of the processingchamber to form a film comprising graphene.
 2. The method of claim 1,wherein the metal substrate comprises a metal foil or a metal filmhaving a metal selected from the group consisting of copper, nickel,ruthenium, rhodium, aluminum, and alloys thereof.
 3. The methodaccording to claim 2, wherein the metal foil or metal film is providedwith a second substrate comprising a metalloid and/or an oxide thereof.4. The method according to claim 3, wherein the metalloid and/or oxidethereof is selected from the group consisting of Si, SiO₂, andcombinations thereof.
 5. The method according to claim 1, wherein anatmosphere of the processing chamber is between about 10⁻⁸ atm and 100atm
 6. The method according to claim 1, wherein contacting the moltenmetal layer with the carbon source gas further comprises contacting themolten metal layer with a secondary gas, the secondary gas beingselected from the group consisting of argon, helium, nitrogen, hydrogen,and mixtures thereof.
 7. The method according to claim 6, wherein thecarbon source gas comprises methane and the secondary gas comprisesnitrogen, argon, and hydrogen.
 8. The method according to claim 7,wherein a flow rate ratio of nitrogen to argon to hydrogen to methaneinto the processing chamber is about 20:60:40:1.
 9. The method accordingto claim 1, wherein a thickness of the molten metal layer is such that asolutal Marangoni number for the metal substrate having the molten metallayer on the top surface thereof is lower than 80, wherein the solutalMarangoni number is:$M_{a}^{s} = \frac{\gamma_{c}d\; \Delta \; c}{\mu \; D_{L}}$wherein: γ_(c) is a solutal surface tension coefficient, Δc is aconcentration gradient across a melt, d is the thickness of the moltenmetal layer, D_(L) is molecular diffusivity in liquid, and μ is dynamicviscosity.
 10. A film comprising graphene prepared by a methodcomprising: providing a metal substrate; continuously advancing themetal substrate into and through a processing chamber comprising one ormore heating elements and one or more gas supply components; heating themetal substrate to form a molten metal layer on a top surface of themetal substrate; contacting the molten metal layer with a carbon sourcegas to form a graphene-comprising layer substantially covering themolten metal layer of the top surface of the metal substrate;solidifying the molten metal layer; advancing the metal substrate havingthe graphene-comprising layer out of the processing chamber; andseparating the graphene-comprising layer to form a film comprisinggraphene.
 11. The film of claim 10, wherein the metal substratecomprises a metal foil or a metal film.
 12. The film of claim 10,wherein the processing chamber comprises: a preheating zone comprisingthe one or more heating elements, a processing zone comprising the oneor more gas supply components, and a cooling zone, wherein the moltenmetal layer is formed as the metal substrate advances through thepreheating zone, the molten metal layer is contacted with the carbonsource gas in the processing zone, and the molten metal layer issolidified in the cooling zone.
 13. The film of claim 10, wherein themetal substrate is advanced from an unwinding roll into the processingchamber.
 14. The film of claim 10, wherein the metal substrate havingthe graphene-comprising layer is wound onto a winding roll after it hasbeen advanced out of the processing chamber.
 15. The film of claim 10,wherein the one or more heating elements comprises at least two heatingelements.
 16. The film of claim 15, wherein the at least two heatingelements comprises a first heating element which heats the top surfaceof the metal substrate and a second heating element which heats a bottomsurface of the metal substrate.
 17. The film of claim 16, wherein thefirst heating element heats the top surface of the metal substrate to nomore than about 50° C. different than a melting point of the top surfaceof the metal substrate.
 18. The film of claim 17, wherein the firstheating element heats the top surface of the metal substrate to atemperature of about 1050° C.±30° C.
 19. The film of claim 16, whereinthe second heating element heats the bottom surface of the metalsubstrate to a temperature that is below a melting point of the bottomsurface of the metal substrate.
 20. The film of claim 19, wherein thesecond heating element heats the bottom surface of the metal substrateto a temperature that is below about 1050° C.